This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2005-10526 filed in Japan on Jan. 18, 2005, the entire contents of which are hereby incorporated by reference.
The present invention relates to a fixing device that can be applied to electrophotographic or electrostatic recording image forming apparatuses or other types of image forming apparatuses.
Conventionally, a fixing device of these types fixes a toner image formed on paper by sandwiching the paper in a nip region between a heating roller and a pressing roller so that the paper is pressed while being heated to fixing temperature, and is disclosed in, for example, JP S60-169876A (hereinafter, referred to as “Patent Document 1”). In Patent Document 1, a plurality of fixing conductions with varied fixing temperatures, pressing forces and the like are prepared, and an appropriate condition is selected among the fixing conditions, depending on the kind of the recording material, and is set. Thus, it is possible to maintain good fixability, regardless of the kind of the recording material.
In recent years, toner containing resin as the main component and having a softening temperature of 50 to 70° C. and a melting temperature of about 120° C. has been used for image forming apparatuses. Therefore, for example, the fixing temperature on the surface of the heating roller is set to 160 to 200° C., and the temperature on the surface of the pressing roller is set to a temperature of not less than 120° C., which is the melting temperature. Thus, the toner on the paper is melted and fixed.
On the other hand, the commercial power supplied to private households has an upper limit of 1500 W (100 V, 15 A). Therefore, the power that the fixing device can consume is limited up to about 700 to 900 W, which affects the processing in the fixing device.
Therefore, there is an attempt to use a heating roller and a pressing roller made of aluminum or iron and having a surface layer with a thickness of about 0.15 to 2.0 mm, instead of the conventional heating roller and pressing roller whose surface layers have a thickness of about 5 to 10 mm, in order to improve the thermal conductivity so that the warming-up time of the fixing device can be reduced.
However, even if the thickness of the heating roller and the pressing roller is reduced and the warming-up time of the fixing device is reduced, the fixing process speed and the paper conveying speed are not increased. This is because in a state in which the consumption power of the fixing device is limited, the heat amount for heating and melting the toner image on the paper is also limited, and therefore the number of paper sheets that can be subjected to fixing process per unit time is limited so that the upper limits of the fixing process speed and the paper conveying speed are spontaneously determined.
Furthermore, even if the upper limits of the fixing process speed and the paper conveying speed are determined, when a large number of paper sheets are subjected to fixing process continuously, the heat of the heating roller and the pressing roller is taken away by the paper or the toner, so that the temperatures of the heating roller and the pressing roller are reduced, and the toner on the paper sheets is not heated sufficiently. At this time, even if most of the toner on the paper is fixed to the paper, the toner that is in direct contact with the surface of the heating roller tends to be attached to the heating roller. The residual toner that is attached to the heating roller is melted on the surface of the heating roller before reaching again the nip region through rotation of the heating roller, and is attached to a peripheral member such as a paper remover or a temperature sensor so that the peripheral member is soiled or the toner is transferred to the pressing roller.
Therefore, a cleaning roller is brought in contact with the heating roller or the pressing roller so that the residual toner on the heating roller and the pressing roller is transferred to the cleaning roller to clean the heating roller and the pressing roller.
However, when the temperature of the heating roller is reduced, the temperature of the pressing roller is further reduced. In this state, the residual toner of the pressing roller is not uniformly attached onto the circumferential surface of the cleaning roller, and the residual toner is attached in the granular form non-uniformly onto the circumferential surface of the cleaning roller.
When the residual toner is attached in the granular form non-uniformly onto the circumferential surface of the cleaning roller, the residual toner tends to be detached and fall off from the cleaning roller due to even the smallest vibration or impact. As a result, the detached and fallen residual toner can soil the paper or the peripheral member. Alternatively, the residual toner is reverse-transferred from the cleaning roller to the pressing roller, and the reverse-transferred residual toner soils the paper.
In order to solve such a problem of the cleaning roller for the pressing roller, it can be thought to perform fine temperature management of the fixing device. However, conventionally, it can only be performed to set selectively a plurality of fixing conditions with varied fixing temperatures or pressing forces as shown in Patent Document 1, and reduction in the temperatures of the heating roller and the pressing roller is not taken into consideration, so that this problem remains unsolved.
Therefore, the present invention is carried out in view of the above-described conventional problem, and it is an object of the present invention to provide a fixing device that can allow toner to be attached uniformly onto the circumferential surface of the cleaning roller, and prevents toner from being detached from the cleaning roller.